1. Field of the Invention
This invention relates to a projection exposure apparatus and a semiconductor-device manufacturing method which uses the apparatus, and more particularly, to an apparatus and a method in which an electronic circuit pattern on a reticle is projected onto a wafer by a projection optical system (a projection lens) in a process for manufacturing semiconductor devices, such as IC's (integrated circuits), LSI's (large-scale integrated circuits), or the like. Adjustments are made to compensate for a change in the optical performance of a lens in the projection optical system due to a change in the optical characteristics of the lens caused by a temperature increase of the lens resulting from absorption of exposure light, whereby a precise projecion pattern image can be obtained.
2. Description of the Background Art
Projection exposure apparatuses (steppers) in which high resolution and high throughput can be relatively easily obtained have been generally used for manufacturing semiconductor devices, such as IC's, LSI's or the like. Such a projection exposure apparatus does not use a batch exposure method in which a pattern image is formed on the entire surface of a wafer by a single exposing operation, but instead uses a so-called step-and-repeat exposure method in which regions are sequentially exposed by moving the wafer after each single exposing operation has been completed, and images of the same pattern are thereby formed on the entire surface of the wafer by repeating the exposing operation a plurality of times.
During the exposure operation, a projection optical system projects a reduced image of an electronic circuit pattern which is formed on the surface of a reticle onto the surface of the wafer with a predetermined projection magnification of, for example, 1/5 or 1/10. In recent projection exposure apparatuses, in response to a tendency toward high integration of semiconductor devices, there has been an effort to reduce the wavelength of exposure light, and to increase the NA (numerical aperture) of the projection optical system of the apparatus.
In general, if the NA of the projection optical system is intended to increase, the depth of the focus of the system is reduced. However, in order to provide a fine circuit pattern, it is necessary to precisely align a reticle with a wafer.
In particular, if the integration of semiconductor devices in a projection exposure apparatus is to be increased, the following problems arise in the projection optical system of the apparatus.
(a) A change in the focus position of the projection optical system.
Lenses constituting the projection optical system of the apparatus absorb part of the exposure light which causes a thermal change, whereby the focus position changes over a period of time, for example, as shown in FIG. 7. FIG. 9 shows a state in which a light beam 51 indicated by solid lines changes to a light beam 52 indicated by broken lines during an exposure process, whereby the focus position changes.
In general, as shown in FIG. 7, the amount of change of the focus position increases with the lapse of time after the start of exposure, and reaches a steady state at time t1. If the exposure is terminated at time t2, the amount of change of the focus position decreases with the lapse of time, and finally returns to the initial state at time t3. A change in the focus position causes no problem if the amount of the change is small. However, the amount of the change which exceeds the range of the depth of the focus of the projection optical system causes a great problem.
(b) A change in the imaging magnification of the projection optical system.
The lenses constituting the projection optical system of the apparatus absorb part of the exposure light which causes a thermal change, whereby the imaging magnification changes over a period of time, for example, as shown in FIG. 8. FIG. 10 shows a state in which a pattern 53 indicated by solid lines changes to a pattern 54 indicated by broken lines, that is, the imaging magnification changes over a period of time.
In general, as shown in FIG. 8, if exposure is started at time t0, the amount of change of the imaging magnification increases with the lapse of time, and reaches a steady state at time t1. If the exposure is terminated at time t2, the amount of change of the imaging magnification decreases with the lapse of time, and finally returns to the initial state at time t3. Such a change in the imaging magnification directly influences an alignment error of a circuit pattern, causing a great problem when a fine circuit pattern is being printed.
Conventionally, regarding the problem of a change in the focus position described in item (a), the change in the focus position is corrected by adjusting the distance between a wafer and the projection optical system in accordance with the change in focus position by moving a wafer stage which contains the wafer along the direction of the optical axis of the projection optical system.
Regarding the problem of a change in the imaging magnification described in item (b), the change in the imaging magnification is corrected by forming an enclosed space in an interval between lenses, and adjusting the air pressure of the enclosed space in accordance with the change in the imaging magnification.
In another approach, the interval between a reticle and a projection lens, or the interval between respective lenses constituting the projection lens, is adjusted in accordance with a change in the imaging magnification.
In a conventional projection exposure apparatus, changes in the focus position and the imaging magnification of the projection optical system of the apparatus are corrected according to the following processes.
Before an actual pattern printing (pattern projection and exposure) operation, measurements are taken to determine how the optical characteristics of the system, i.e., the focus position and the imaging magnification, change with the lapse of time after the start of exposure. Data obtained by sampling the measured data at several points in time, or by approximating the measured data with a certain function, are stored in memory means.
When a pattern printing operation is actually performed, the necessary amounts of correction of the focus position and the imaging magnification over a period of time are calculated using the data from the memory means in accordance with the lapse of time after the start of exposure, whereby the focus position and the imaging magnification are corrected.
Such an approach, however, has the problem that a correction error occurs if conditions when the correction data are obtained and stored in the memory means differ from actual pattern printing conditions. For example, if a difference is present in the intensity of exposure light or in the transmittance of exposure light for the reticle, such a difference causes a correction error.
In another approach, a temperature change in the projection optical system is measured by a temperature sensor, and changes in the optical characteristics of the system are corrected utilizing an output signal from the temperature sensor. In such an approach, however, changes in the optical characteristics of the projection optical system are indirectly estimated from the temperature change, and therefore, the obtained results are not reliable.